1. Field of the Invention
The present invention relates to a hydraulic tensioner for a timing mechanism included in an internal combustion engine, and a timing mechanism covering structure serving as one of structural members defining a timing mechanism chamber containing a timing mechanism and a lubricating system included in an internal combustion engine. More specifically, the present invention relates to a hydraulic tensioner for a timing mechanism included in an internal combustion engine, having a reduced number of parts, having improved response characteristic and capable of being readily manufactured, and to a timing mechanism covering structure for an internal combustion engine, capable of facilitating maintenance work for the maintenance of a tensioner, of holding the component members of accessories and of preventing support shafts from falling off.
2. Description of the Related Art
As stated in JP-A No. 5-99288, in a conventional internal combustion engine, an open end of the main gallery of the lubricating system of an internal combustion engine on the side of a timing mechanism chamber containing a timing mechanism is stopped with a plug. A tensioning unit included in a hydraulic tensioner for exerting pressure to a timing chain included in a timing mechanism for transmitting the rotation of a crankshaft to a camshaft in a direction substantially perpendicular to the running direction of the timing chain is disposed at a position separate from the main gallery, and working fluid is supplied from the main gallery through a separate oil supply passage to the tensioning unit. Therefore, an additional member, i.e., the plug, is necessary to stop the open end of the main gallery on the side of the timing mechanism chamber, and a wall in which the open end of the main gallery opens must be tapped by a tapping process to close the open end of the main gallery with the plug. Since the tensioning unit is disposed apart from the main gallery, the oil supply passage connecting an oil pump to the tensioning unit is long and it is difficult to raise the working pressure in the tensioning unit quickly immediately after the start of the internal combustion engine. Thus, problems resides in the response characteristic of the tensioner. Since the tensioner is unable to function satisfactorily, the timing chain of the timing mechanism slackens, vibrates and generates noise. Work for forming the long oil supply passage is difficult.
It is difficult to form a narrow bore in a cylinder block owing to the arrangement of an assembly line. Therefore, a pressure regulating constriction cannot be formed and the working fluid of a regulated pressure cannot be supplied to the tensioner.
In an internal combustion engine disclosed in JP-A No. 7-42571, a timing mechanism chamber is formed by covering the respective end walls of a cylinder block and a cylinder head with a timing mechanism cover, and a timing mechanism including a timing chain and a crankshaft sprocket is arranged in the timing mechanism chamber. An end part of the crankshaft penetrates and projects from the timing mechanism cover, and an accessory driving pulley is fixedly mounted on the end part of the crankshaft projecting from the timing mechanism cover. A handhole formed in the timing mechanism cover for the maintenance of an oil pump disposed in the timing mechanism chamber is covered with a cover. The cover is placed on the timing mechanism cover with bolt holes formed therein aligned with threaded holes formed in the timing mechanism cover, and is fastened to the timing mechanism cover with bolts. Since the bolt holes formed in a peripheral part of the cover are concealed behind a crankshaft pulley and an accessory driving belt wound around the crankshaft pulley, the accessory driving belt needs to be removed from the crankshaft pulley and then the crankshaft pulley needs to be removed from the crankshaft before removing the cover for maintenance work, which is very troublesome.
FIG. 24 is a sectional view of a front end part of a V-shape internal combustion engine 01 provided with another prior art timing mechanism cover structure disclosed in JP-A No. 7-42571. Referring to FIG. 24, a chain cover (timing mechanism cover) 03 is fastened to the front end of a cylinder block 02 included in the V-shape internal combustion engine 01 to form a timing mechanism chamber 04. Disposed in the timing mechanism chamber 04 is a timing mechanism comprising a crankshaft sprocket (drive sprocket) 06 mounted on a crankshaft 05, and a chain 07 wound around the crankshaft sprocket 06 and a camshaft sprocket (driven sprocket), not shown, mounted on a camshaft, not shown. The chain 07 extended between the crankshaft sprocket 06 and the camshaft sprocket is wound round a pump sprocket 09 mounted on the shaft of a water pump 010. The rotation of the crankshaft 05 is transmitted through the chain to the camshaft to drive the camshaft for rotation and to the shaft of the water pump 010 to drive the water pump 010.
A pump housing 011 is fastened together with a rear chain cover 012 to the cylinder block 02 with bolts 015. A handhole for maintenance work is formed in a part of the chain cover 03 facing the water pump 010 and is covered with a cover 016. Sometimes, the bolts 015 are caused to become loose by vibrations or the like. It is possible that the bolts 015 loosen greatly when the distance between the heads of the bolts 015 and the cover 016 disposed opposite to the bolts 015 is long as shown in FIG. 24. Consequently, the pump sprocket 09 becomes unsteady, the chain 07 vibrates, and a chain guide and a tensioner arm hit frequently on the chain cover 03 to generate noise. If, by any chance, the bolts 015 should come off the cylinder block 02, the water pump 010 cannot be held in place and would cause serious damage. If the bolts 015 comes off the cylinder block 02 and need to be recovered, the chain cover 03 needs to be removed, and the crankshaft pulley 017 needs to be removed from the crankshaft 05 to remove the chain cover 03, which requires very troublesome work.
If the pivot pin of a tensioner is made of a steel, an arm body is made of an aluminum alloy, and the pivot pin is fitted in a mating hole formed in the arm body in a press fit, the interference between the pivot pin and the arm body tends to decrease at a high temperature due to the difference in coefficient of thermal expansion between the pivot pin and the arm body, and it is possible that the pivot pin comes off the arm body.